A ceiling fan of a type hitherto known has fan blades sandwiched and fixed between a supporting frame and a rotor as disclosed, for instance, in Patent Literature 1.
Referring to FIG. 15A and FIG. 15B description is provided hereinafter of a conventional ceiling fan. FIG. 15A is a cross sectional view showing a main part of the conventional ceiling fan and FIG. 15B is a plan view showing a part of a fan blade of the ceiling fan.
As shown in these figures, the conventional ceiling fan comprises disc-like stator 103 provided with shaft 104 inserted in and fixed to the center thereof, and annular rotor 105 disposed in close proximity to the periphery of stator 103. The conventional ceiling fan also comprises cap-shaped upper supporting frame 101 and lower supporting frame 102 fixed to the top and bottom of rotor 105 and rotatably supported by shaft 104, and a plurality of fan blades 115 disposed radially by having their one ends inserted into a plurality of slots provided around the periphery of upper supporting frame 101.
Fan blades 115 are inserted between upper supporting frame 101 and rotor 105 when they are fixed to rotor 105. This structure helps retain fan blades 115 temporarily and facilitate the fixing work more conveniently. The structure can also hold fan blades 115 robustly against a load in the vertical direction since fixed portions of fan blades 115 have many areas of contact in their upper and lower surfaces after they are fixed to rotor 105.
As described, the conventional ceiling fan has the structure designed to fix fan blades 115 directly to rotor 105 such that it does not require any special member for mounting fan blades 115 to rotor 105.
When using fan blades 115 made specifically of a metallic material, however, it becomes necessary to lighten a weight of metallic fan blades 115 by reducing their thickness, and this consequently weakens their rigidity and strength. This gives rise to a possibility that fan blades 115 become damaged due to metal fatigue resulting from repeated imposition of the load during a long term of use since stresses concentrate on root portions of fan blades 115 as a reaction of the air being thrust downward.
It is therefore necessary for the conventional ceiling fan of the above kind to have fan blades 115 of such a shape that is not easily damaged in the root portions, and a structure for preventing fan blades 115 from coming off rotor 105 even if they are broken at the root portions.
On the other hand, there is another type of conventional ceiling fan disclosed in Patent Literature 2, which is designed to ensure safety even when fan blades are broken in the proximity of their root portions.
Description is provided hereinafter of the ceiling fan with reference to FIG. 16. FIG. 16 is an exploded perspective view showing a fan blade portion and a retaining member of the conventional ceiling fan as they are being assembled.
As shown in FIG. 16, the conventional ceiling fan comprises cowling 120, which is rotatable with a rotor (not shown), and fan blades 121 attached to cowling 120 by means of retaining members 123. Retaining members 123 and fan blades 121 are mounted to cowling 120 with screws while one ends 123a of retaining members 123 are held in alignment with mounting portions 121a of fan blades 121. Each of retaining members 123 is formed of a long strip of metal plate extending from one end 123a to other end 123b in a direction of the distal end of fan blade 121, and the other end 123b is fastened to the surface of fan blade 121 at a point of a given distance away from the one end 123a. While an area adjacent to mounting portion 121a of fan blade 121 is liable to gradual deterioration due to fatigue with time, a separated piece of fan blade 121 is kept connected to cowling 120 with retaining member 123 even when any of fan blades 121 is broken near its one end 123a. This structure can thus prevent fan blades 121 from separating from the main unit and falling below, thereby ensuring safety in the surrounding area.
The conventional ceiling fan of this kind has a comparatively small load exerted on retaining members 123 when a synthetic resin is used for the material of fan blades 121. A problem exists, however, in the case of using fan blade assemblies, each consisting of a metallic blade frame and a metallic fan-blade plate. That is, the metallic fan blade assemblies are generally composed of fan-blade plates of a small thickness and blade frames of a thickness larger than the fan-blade plates in order to reduce their weight. As the thickness of the blade frames is decreased in an attempt to reduce the material and the weight, however, the blade frames lose their physical strength. This may lead to breakage of the fan blades due to metal fatigue caused by repeated imposition of load during a long term of use since stresses developed by the blade surfaces to downwardly thrust the air act upon the blade frames via the fan-blade plates and concentrate on their root portions. It therefore becomes necessary to provide some measures for preventing broken pieces of the fan blade assemblies from coming off the main unit even when using the fan blade assemblies consisting of the blade frames and the fan-blade plates.    Patent Literature 1: Japanese Patent Unexamined Publication, No. 1990-188693    Patent Literature 2: Japanese Patent, No. 3,803,475